This invention relates generally to optical data storage and retrieval where multiple bit information is stored at each physical location, and more particularly to high density optical storage in which the absorption spectrum of a chemical composition at a position on an optical disk conforms to a desired value in multiple bit code for that position.
Optical recording technology currently uses a disk to store large amounts of binary-coded information in a small space by using pits and intervening spaces, or lands. Typically, a highly focused diode laser beam passes over a reflective surface. The reflected light is directed to a sensor that detects modulations that result when a vertical transition from a land to a pit, or vice versa, diffracts rather than reflects the light beam. The result is a binary code with the transitions equalling one""s and the variable spaces between them equalling some number of zero""s.
In conventional optical disks, because there cannot be adjacent vertical transitions, and in fact transitions need to be far enough apart for the reading beam to distinguish between them, encoding of the binary information is required whereby a greater number of physical locations are needed to store an equivalent number of bits. The insertion of error detection and correction requires further encoding which occupies storage locations. The additional levels of coding reduce both the amount of information that can be stored on the disk and the rate at which information can be retrieved. As a result, despite their less than one micrometer detail, optical disks typically require several micrometers to record one byte of data.
Efforts to increase the data retrieval rate have included multichannel reading (see U.S. Pat. No. 5,195,152). Multichannel reading increases data retrieval speed but not storage efficiency or capacity. If the underlying problem of inefficient disk storage were resolved, the speed increases resulting from multichannel reading would be significantly enhanced.
Efforts to increase the data storage capacity have included lower wavelength laser diodes, superresolution, multi-layered or holographic recording, double-sided disks and data compression. These methods all promise increased capacity, but not greater storage efficiency or retrieval speed.
Efforts to increase storage efficiency have included spectral hole burning (see U.S. Pat. No. 5,231,626). This method suffers from serious disadvantages because the storage of the information is not stable with the passage of time.
This invention addresses all three of the above identified issuesxe2x80x94storage density, storage efficiency and retrieval speedxe2x80x94by giving the storage medium a stable multibit value at each position on the disk. By replacing the lands and pits of conventional disks with dots that absorb different wavelengths, this device allows the storage and retrieval of information by multiple bits rather than by a single bit. Specifically, the preferred embodiments of the apparatus use a plurality of light sources and a plurality of detectors for reading information stored as multiple-colored dots. Storage density increases depend on the number of colors used. For example, if 256 compositions with different absorption properties were used, each dot could represent 8 bits of data. Retrieval speed increases result from the fact that data is transmitted in multiple-bit bytes rather than one bit at a time. Further objects and advantages of the invention will become apparent from a consideration of the drawings and description.